The present invention relates to laser-beam machining or processing of a printed circuit board. More particularly, the invention relates to improvement of the method and the apparatus disclosed in U.S. Pat. No. 5,010,232. The patent was issued to the inventor of the present invention and to another on Apr. 23, 1991.
The density of an integrated circuit is increased and coincidentally, the number of external connection terminals per integrated circuit is also steadily increased. A printed circuit board for mounting of high-density electronic parts or devices with a lot of external connection terminals cannot be provided with a necessary number of external connection terminals by the conventional planar mounting method in which lead connection terminal pads are arranged in a row around a mounted part on an outer layer of the circuit board. In such a printed circuit board, it is necessary to decrease the width of each connection terminal and the connection terminal interval, which will be referred to as pitch decreasing hereafter. Moreover, it is required to provide connection terminals not only on the outer layer but also on an inner layer of the circuit board. Several types of spot facing methods have conventionally been known, which are performed to expose the inner layer of a printed circuit board. For example, there is a method of directly applying a laser beam to insulating layers of the circuit board. This method exposes a target inner copper-foil layer by removing all the insulating layers between the outer layer surface and the inner layer by a laser beam. Moreover, there is a method as disclosed in Japanese Patent Unexamined Publication No. 2-262941. According to this method, an inner copper foil is exposed by mechanically detecting a substrate surface position to obtain a substrate thickness error and by carrying out spot facing up to a predetermined depth by a cutter of Z axis of the main axis in accordance with constant-rate-corrected inner-layer positional data.
U.S. Pat. No. 5,010,232 discloses a bottomed-hole forming method which is performed utilizing cutting and laser-beam processing in combination. First, an outer copper foil, copper foil layers and insulating layers are removed by a cutter to an extent immediately before a target inner copper foil. The remaining insulating layer is then removed by a laser beam to expose the target inner copper foil.
To decrease the widths of patterns and achieve the pitch decreasing in a printed circuit board, a thinner copper-foil layer is more advantageous because the resolution is improved when forming a pattern. A printed circuit board is formed by heat-pressing and laminating several to tens of plates in a laminating process. In the heat-pressing process, a resin flows out of peripheral ends of the printed circuit board. Therefore, the plate thickness at the peripheral ends tends to be thinner, up to approx. 0.1 mm, than the thickness at the central portion. Moreover, where an insulating layer between copper-foil layers has a thickness of 0.1-0.2 mm, an error in this thickness can reach approx. 0.03 mm (30 .mu.m), and the yield of products is greatly decreased when performing spot facing directly by a cutter. Accordingly, it cannot be avoided to set the thickness of an inner copper-foil layer to 60 .mu.m in view of the fluctuation of 30 .mu.m of the layer thickness and thus, it is impossible to form a thin pattern.
When processing a printed circuit board directly by a laser-beam machining method, insulating layers which are made of a resin and glass-fiber cloth and have a thickness of up to 1 mm have to be removed. A high-output energy density is necessary to remove the resin and glass-fiber cloth and the removal depth per pulse is approx. 1 .mu.m at most. Therefore, to finish a square with one side of 20 mm or an area of 400 mm.sup.2, irradiation of a total of 20,000 pulses is required even when using an excimer laser beam of the following specifications. That is, in this laser beam, the high output is reduced to 1/5, in other words, the irradiation area is expanded up to 5 times, the simultaneous perforation area is 5 mm.times.4 mm or 20 mm.sup.2, the beam output at the exit of a generator is 500 mJ/cm.sup.2, and the beam size is 5 mm.times.20 mm. If the printed circuit board is processed at a pulse frequency of 100 Hz and has a relatively small influence of heat, the processing time of at least 200 sec is necessary. Moreover, because the copper foil surface is greatly oxidized and the resin at a boundary of the processed area is greatly carbonized, the printed circuit board is not finished for practical use. The method disclosed in Japanese Patent Unexamined Publication No. 2-262941 detects the height of the surface of a printed circuit board and corrects the plate thickness error. However, because the cutting depth easily becomes excessively large when the printed circuit board is perforated at a time, the method requires two processes of roughing and finishing. Furthermore, there are cases where printed circuit board surface detection and finishing are inevitably repeated. Therefore, the time of completion of spot facing becomes too long and, for example, it takes 90 to 150 sec to finish a 20 mm-square. Conventionally, the roughing rate of a 2.0 mm-cutter is 2.0 m/sec and the finishing rate thereof is 0.5 m/sec. However, it is difficult to uniformly finish a spot-faced portion due to fluctuation of the depth of an inner layer and inclination of the inner layer. There is a possibility that, depending on the setting of a cutting depth, a residual portion which cannot be perforated at all will be produced or the inner layer will be cut off in a printed circuit board. Further, a cutter when abraded may cause burrs or exfoliation in the inner layer of a portion perforated. Therefore, the yield of products decreases and moreover, the running cost is large because the cutter end is greatly abraded due to cutting of the copper foil of the inner layer.
The method of U.S. Pat. No. 5,010,232 assumes that the processing with a cutter and a laser beam is performed before plating, patterning and solder-masking processes. When performing the spot facing before the plating, patterning and solder-masking processes, a practical difficulty is involved because it is required to prevent solder from attaching to a spot-faced portion and to protect the portion from etching. Therefore, it is necessary to provide a processing method with a high reliability which can be executed after plating, patterning, and solder-masking.